Pass elements generally represent the largest semiconductor area of AC/DC converters, DC/AC inverters and high voltage DC/DC converters. For that reason a lot of effort has been spent optimizing the figures of merit (such as Rdson or gate charge) for pass elements in various applications, but that effort has primarily been focused upon metal-oxide-semiconductor field-effect transistor (MOSFET), insulated-gate bipolar transistor (IGBT), and BIPOLAR transistor optimization.
Thyristors have long been accepted as the highest current density devices available, however, thyristor use has been limited in switching power supplies due to their slow turn off capabilities, and little work has been done to utilize such devices in lower power (<1000 W) applications with fast switching frequencies (>20 kHz). In addition, most commercially available thyristors come in a silicon-controlled rectifier (SCR) form and cannot be actively turned off and are complex to drive.
Metal-oxide-semiconductor (MOS) controlled thyristors (MCT) are devices with a high impedance gate structure which can be turned on and off utilizing a voltage on the high impedance gate. An equivalent schematic is shown in FIG. 1. The device works by injecting carriers into an SCR latch structure and then removing those carriers by shorting one of the base-emitter junctions of the equivalent bipolar transistors in the SCR latch structure equivalent schematic.
There are six significant issues which have reduced the use of gate turn off thyristors in lower current, switching power supply applications:
1. Slow turn off speed—typically >600 ns even for 600V devices. Switching AC/DC converters need to be <250 ns to start to be considered for use.
2. Maximum turn off current—if exceeded the device cannot be turned off.
3. Requirement for a plus/minus gate drive, requiring the creation of a negative rail.
4. The perception that the device could “latch on” from noise and therefore is not reliable.
5. Thyristors generally have no gate controlled current saturation region to control the turn on and turn off commutation. Thyristors also have an asynchronous on and off time with the turn on time being far quicker than the turn off time. IGBT, MOSFETs, and BIPOLARs transistors have a gate voltage controlled current saturation region between fully on and fully off, where the voltage on the gate can be used to control the turn on characteristics of the switch. Thus thyristors must be designed for a specific turn on characteristic limiting the applications of any one device, or expensive and potentially lossy snubbers must be incorporated (to reduce electromagnetic interference (EMI), prevent overvoltage, and improve efficiency compared to unsnubbered implementations). As snubbers often would not be required in low current applications, this makes gated thyristors far less attractive.6. Customers are experienced with IGBTs, BIPOLARs and MOSFETs, and a considerable number of drivers, test data, application notes are available for their use, and any differences to what customers are used to are usually not tolerated.
It would therefore be desirable to produce a gated thyristor that overcomes the aforementioned problems, with a resulting device that would utilize less silicon area than an IGBT, BIPOLAR or MOSFET sized for the same application. In addition, it would be desirable to have a thyristor with a lower gate charge, and a lower forward drop for a given current density. Furthermore, it would be desirable to produce a gated thyristor where once triggered the latch structure does not have the same Cgd or Ccb capacitor that must be charged from the gate, and therefore the desired gated thyristor has the potential to be both cheaper, require a smaller gate driver, and take up less space than standard solutions.